Theorists have long proposed that negative index materials, which bend light opposite to what is expected from Snell’s law, would have all sorts of unusual optical properties, and could be the basis of a “perfect lens.” Experiments on metamaterials (which are arrays of engineered miniature structures), and photonic band gap materials (which have periodically varying dielectric properties) are now providing strong evidence that it is possible to make negative index materials.

Similar ideas have been applied to acoustics. For example, phononic materials, the analogue to photonic materials, have a spatially modulated mass or elasticity and can be tailored to have a negative dynamic modulus and mass (the acoustic equivalent of an index of refraction) for focusing sound waves. Writing in Physical Review Letters, Shu Zhang, Leilei Yin, and Nicholas Fang at the University of Illinois at Urbana-Champaign in the US present the design and test results of an ultrasonic metamaterial lens for focusing 60kHz (∼2cm wavelength) sound waves under water. The lens is made up of subwavelength elements and is therefore potentially more compact than phononic lenses that operate in the same frequency range.

The lens consists of a network of fluid-filled cavities called Helmholtz resonators that oscillate at certain sonic frequencies. Similar to a network of inductors and capacitors in an optical metamaterial, the arrangement of Helmholtz cavities designed by Zhang et al. have a negative dynamic modulus for ultrasound waves.

Zhang et al. show that they can focus a point source of sound to a spot size that is roughly the width of half a wavelength and their design may allow them to push the resolution even further. – Jessica Thomas

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